Thinning
Removing bulk without shortening the hair, usually with toothed thinning scissors.
Also: thinning shears, debulking
Thinning means taking away some of the hair's weight while keeping its overall length. Thinning scissors have one straight blade and one with teeth, so each cut removes only some of the hairs in a section. Stylists use them in the mid-lengths of thick hair, to blend clipper work into longer hair, and to soften heavy lines. Too much thinning near the roots can leave short hairs that poke out, so it is used sparingly, especially on curly hair.